The present invention relates to radio communication devices and relates, in particular, to a radio communication device such as a portable telephone that uses a highly efficient power amplification method in which a multi-carrier signal using a small number of, in particular, about two to three carriers (carrier waves) serves as a transmission modulation wave.
The portable telephones, which have continued achieving remarkable technological progress in recent years, have advanced through the sequential steps of the first generation (analog portable telephone), the second generation (digital portable telephone) and the third generation (IMT-2000) in the past. Then, at the present, the standardization work and the researches and development of the next fourth generation (Beyond IMT-2000) have already started aiming at the practical use in the year of about 2010.
The construction of the RF (radio frequency) circuit of a radio communication device is influenced by the RF electric signal modulation system provided by the standard of the radio communication system. Although the standardization work of the fourth generation has not yet been advanced, a MC/DS-CDMA (Multi Carrier/Direct Spread-Code Division Multiple Access) system recommended by NTT DoCoMo Inc. is regarded as promising as the modulation system of uplink (communication from a portable telephone to the base station (sometimes called the “Upward link”, “Reverse Link” or the like)).
This MC/DS-CDMA system is disclosed in, for example, the Paper No. B-5-72 “Outline of the broadband radio access experiment device” at the general meeting of The Institute of Electronics, Information and Communication Engineers held in 2003 and likewise in the Paper No. B-5-77 “Indoor experiment result of the throughput characteristic in upward link multi-carrier/DS-CDMA broadband radio access” and so on. According to Table 2 in the document B-5-72 and Table 1 in the document B-5-77, it can be understood that a modulation signal of a occupancy bandwidth of 40 MHz in total is provided by arranging two carriers of a DS-CDMA signal (direct sequence code division multiple access signal) of a bandwidth of 20 MHz per carrier (carrier wave) on the frequency axis. FIG. 5 shows this in a schematic spectrograph. A modulation wave 501 due to a first carrier and a modulation wave 502 due to a second carrier are adjacently arranged on the frequency axis. In the aforementioned MC/DS-CDMA system, each of the two carriers is a modulation wave of a bandwidth of 20 MHz, and the two carriers occupy a total bandwidth of 40 MHz.
As described above, the MC/DS-CDMA system uses a multi-carrier signal that has a plurality of carriers although the number of the carriers is only two. Moreover, it can be understood that linear amplification is required since each of the carriers is a DS-CDMA signal.
Moreover, FIG. 1 (2) in the aforementioned document B-5-72 shows a schematic block diagram of the radio communication terminal at which this MC/DS-CDMA system is used as a transmission modulation system. FIG. 7 of the present specification shows a diagram obtained by rearranging the diagram in this document by eliminating the portions unnecessary for the explanation of the present invention.
In FIG. 7, a transmission modulation wave generated by a signal generator 701 is processed (subjected to amplification, frequency conversion, band limiting and so on) by a transmitter circuit 702 and transmitted from an antenna element 704 or an antenna element 705 through a TX amplifier and a duplexer 703. Conversely, a reception modulation wave received by the antenna element 704 or the antenna element 705 is processed (subjected to amplification, frequency conversion, unnecessary frequency component removal and so on) by a receiver circuit 706 through a RX amplifier circuit and a duplexer 703 and demodulated in a demodulator 707.
In the radio communication terminal of a portable telephone or the like, the power consumption of the TX amplifier used immediately before the antenna of the transmission system, i.e., a power amplifier is an extremely serious problem. The power amplifier is a circuit that operates to amplify the modulation signal of a minute electric power on the milliwatt order produced in the signal processing section to a high power on the watt order at a stroke and send the resulting power to the transmission antenna. In general, the power amplifier, which handles the high power on the watt order, therefore becomes a part of markedly great power consumption among all the electronic parts. Therefore, it becomes extremely important to reduce the power consumption and increase the efficiency of particularly the power amplifier for the purpose of reducing the power consumption of the radio communication terminal.
If the efficiency of the power amplifier is high, the continuous duration of telephone conversation of the portable telephone can be made long with respect to the battery of the same capacity. Moreover, if a margin is consequently produced in the continuous duration of telephone conversation, the capacity of the battery can be reduced, and the portable telephone is allowed to totally have a light weight, a small size and a low cost. As described above, increasing the efficiency of the power amplifier is the serious matter directly connected to the marketability and the convenience of the portable telephone.
As the circuit configuration of a power amplifier for a multi-carrier signal as in FIG. 5, there are known the two methods of the “collective amplification system” shown in FIG. 8 and the “individual amplification system” shown in FIG. 9. A difference between the collective amplification system and the individual amplification system is described in detail in, for example, the “prior art” section of JP 2000-68958 A.
FIG. 8, which shows the collective amplification system, is obtained by rearranging FIG. 12 of the aforementioned first patent document by eliminating the portions that are not related to the essence of the present invention. A plurality of carriers (carrier waves) individually generated by a plurality of signal generators 801 through 804 are first bundled into one signal line first by a combiner 805, collectively amplified by a power amplifier 806 and transmitted from an antenna element 807. The collective amplification system is the system generally adopted by the home electric appliances since the circuit configuration is simple, small-sized and low cost. For example, in a wireless LAN product of the IEEE802.11a Standard for personal computers, an OFDM signal, which is a typical multi-carrier signal, is amplified by the collective amplification system.
However, when the multi-carrier signal is amplified by the collective amplification system, there is a widely known problem that the efficiency of the power amplifier is significantly deteriorated and the power consumption increases. This is ascribed to the fact that the plurality of carriers are momentarily mutually combined to intensify or conversely canceled to weaken and the amplitude fluctuation of the signals consequently becomes remarkable. This problem is described in many documents including, for example, the “prior art” section of JP 09-149090 A.
Therefore, it is also attempted to use the individual amplification system of FIG. 9 in place of the collective amplification system of FIG. 8 in order to improve the efficiency of the power amplifier. FIG. 9, which shows the individual amplification system, is obtained by rearranging FIG. 11 of the aforementioned first patent document by eliminating the portions that are not related to the essence of the present invention. A plurality of carriers (carrier waves) individually generated by a plurality of signal generators 901 through 904 are individually amplified as they are by a plurality of power amplifiers 905 through 908 and subsequently bundled into one multi-carrier signal by a power combiner 913. The power combiner 913 is concretely provided by isolator circuits 909 through 912, a resistance matching circuit or the like for preventing the interference between the power amplifiers 905 through 908 due to signal reflection. Then, the multi-carrier signal, which is obtained by combining the electric powers, is transmitted from an antenna element 914.
The individual amplification system as in FIG. 9 is considered to have the possibility of increasing the efficiency of the power amplifier in comparison with the collective amplification system as in FIG. 8. The above is because the amplitude fluctuation is suppressed low in the individual amplification system (FIG. 9) since the signals are mere single-carrier signals when the signals pass through the individual amplifiers 905 through 908 even though the final output is a multi-carrier signal.
However, there have been several technical problems in actually achieving the individual amplification system (FIG. 9) at the home electric appliance level, and above all, a big problem has been the combiner. For example, in the case of the power combiner 913 of FIG. 9, there cannot be avoided increases in size and cost due to the use of a number of isolator parts and a power loss due to resistors. In particular, the loss due to the resistors means that the power amplified with effort is partially uselessly consumed even though the power amplifiers 905 through 908 can achieve highly efficient amplification, and this might conversely lead to an increase in the power consumption after all.
The reason why the circuit generating a loss due to the resistors must be used as the combiner 913 is that the multi-carrier signal (FIG. 5) supposed by the present invention is the signal in the unfavorable conditions in which the two carriers 501 and 502 are adjacent to each other without any guardband on the frequency axis. There is a known means that can achieve power combining in a loss-less manner while preventing the mutual interference by securing isolation between terminals if a guardband exists between the two carriers 501 and 502 and their frequency bands are separated apart from each other. For example, the duplexer circuit is the typical example of the loss-less combiner in such the favorable conditions. However, in the case of the unfavorable conditions in which almost no guardband exists as in FIG. 5, there is known no implementation method for achieving the loss-less combiner that can easily be provided on a circuit board.
As a prior art countermeasure against this problem, for example, the aforementioned first patent document insists that the isolator circuits 909 through 912 of the power combiner 913 of FIG. 9 can be eliminated by using a variable filter circuit or in a similar manner. However, even this first patent document cannot propose a means for achieving the loss-less configuration of the matching circuit that employs resistors located next to the isolator circuits.
As described above, since the power combiner 913 is required to meet the intense demand, it has been practically difficult to provide the individual amplification system (FIG. 9).